If you are using MS Word as your editor in Outlook and have enabled the spell check as you type option, I am sure that must have noticed that the subject is not spell checked as you type. To have that happen automatically, you must enable the option to Check spelling before sending.
Follow these steps to make that happen:
- Click on Tools | Options | Spelling.
- Select Always Check Spelling before Sending.
This setting is, by default, turned off, so you will have to turn it on if you want your subject spell checked automatically.
Clicking F7 to start a spell check will also check the subject line.